As an optical circuit chip used generally, there is an optical circuit chip having a silica-based optical waveguide which uses silica-based material for both a core and a clad included in an optical waveguide. The relative refractive index difference of the core and the clad is as small as about 0.3% in the silica-based optical waveguide. Therefore, even if the waveguide size is large, single mode conditions are easily satisfied, and the mode field diameter is generally about 5 to 10 μm.
By the way, when an optical circuit chip is connected to another optical component, a form interposing an optical fiber is taken in many cases in order to expand the versatility. The mode field diameter of a commonly-used single mode optical fiber is approximately 9 to 10 μm in the wavelength region of 1.3 μm band and 1.55 μm band, which is close to the mode field diameter of the silica-based optical waveguide. Therefore, it is possible to obtain a low-loss optical coupling between the optical circuit chip having the silica-based optical waveguide and the single mode optical fiber.
On the one hand, in recent years, a research on an optical waveguide with a high refractive index difference is active in which the relative refractive index difference between the core and the clad is increased much larger than that of the silica-based optical waveguide by using, as the core, a high refractive index semiconductor such as silicon and gallium indium arsenide phosphorus. The bend radius of the high refractive index difference optical waveguide is generally equal to or shorter than 100 μm and is a few orders of magnitude less than that of the silica-based waveguide. Therefore, if the high refractive index difference optical waveguide is used, the size of the circuit chip becomes smaller dramatically, which enables significant high-density integration. In particular, in the optical circuit chip using an SOI (Silicon on Insulator) substrate having a silicon oxide film layer and a silicon layer on a substrate, it is possible to utilize a low refractive index silicon oxide film layer as a clad and the high refractive index silicon layer as a core. Therefore, it is possible to divert fine processing technologies and production facilities for silicon processes used for producing an LSI (Large Scale Integration) to the production of the optical circuit chip having the SOI substrate, and its commercialization is highly anticipated.
In the optical circuit chip including such high refractive index difference optical waveguide, it is a connection by means of an optical fiber that is used most widely as a connection method to another optical component. However, while the mode field diameter of a commonly-used single mode optical fiber is equal to 9 to 10 μm, each mode field diameter of most high refractive index difference optical waveguides is equal to several hundred nanometers. Therefore, if the optical fiber is directly connected to the optical circuit chip including the high refractive index difference optical waveguide, a large optical loss is generated because of the large difference between both mode field diameters. As a method for adjusting both mode field diameters to obtain a low-loss optical coupling, there is a method for providing a spot size converter to change a mode field diameter for at least one of an optical circuit chip and an optical fiber. In this case, the spot size converter to expand the mode field diameter is required for the optical circuit chip. On the other hand, the spot size converter to reduce the mode field diameter is required for the optical fiber.
A spot size converter provided for the optical fiber is commonly used in which the core size of the optical fiber gradually decreases toward a connection end to reduce the mode field diameter.
On the other hand, a spot size converter provided for the optical circuit chip is classified into the following two general types. The first type is a spot size converter in which the mode field diameter is expanded by gradually increasing the core size of the optical waveguide toward the connection end of the optical circuit chip. The second type is a spot size converter in which the core size of the optical waveguide is gradually decreased toward a connection end and the core is terminated short of the end face of the optical circuit chip, and the mode field diameter is expanded by transferring the optical energy to the second core formed around the core.
Above-mentioned first type of the spot size converter among the spot size converters provided for the optical circuit chip is described in non-patent literature 1, for example. In this case, by additionally forming a silicon layer about 10 μm thick on an SOI substrate and further processing the thick silicon layer complicatedly, the core size is expanded. Thus, when the core size is expanded, a high refractive index semiconductor to be a core must be formed thick. Therefore, there is a problem that a production cost becomes higher.
Above-mentioned second type of the spot size converter among the spot size converters provided for the optical circuit chip is described in non-patent literature 2, for example. FIG. 1 shows schematic views of a spot size converter 100 described in non-patent literature 2. FIG. 1A is a top view of the spot size converter 100. FIG. 1B is a cross-sectional view taken along the line of FIG. 1A. FIG. 1C is a right side view of FIG. 1A. Although a silicon core 103 and a core reducing part 104 which are shown by a dotted line in FIG. 1A are not actually exposed on an upper surface as shown in FIG. 1B, they are shown in order to make their locations understandable. Much the same is true on the core reducing part 104 shown by a dotted line in FIG. 1C.
In the spot size converter 100 shown in FIG. 1, 200 nm thick of the silicon core 103 on the SOI substrate is processed into a tapered shape. That is to say, the core width is reduced so as to become about 80 nm at the terminal of the silicon core 103 in the direction of the optical propagation. Here, a second core 105 with a cross-sectional surface of 3 μm height by 3 μm width is formed over the core reducing part 104 where the silicon core 103 is made a tapered shape. The second core 105 is made of a material whose refractive index is smaller than that of the silicon core 103 and larger than that of a buried oxide film layer 102, such as a silica-based material. In this case, the optical energy of the light propagating through the silicon core 103 is coupled with and transferred to the waveguide mode in the second core 105 while it is propagating through the core reducing part 104. And the optical energy coupled with the fundamental mode formed in the second core 105 is coupled with a single mode optical fiber.
It is possible to make the spot size converter 100 shown in FIG. 1 in a relatively simple way by means of diverting the manufacturing technology of a silica-based optical waveguide.
Non-patent literature 1: Assia Barkai et al., “Double-Stage Taper for Coupling Between SOI Waveguides and Single-Mode Fiber”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 24, Dec. 15 and 2008, pp. 3860-3865
Non-patent literature 2: Tai Tsuchizawa et al., “Microphotonics Devices Based on Silicon Microfabrication Technology”, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 11, NO. 1, January/February 2005, pp. 232-240